This invention relates to spark ignited internal combustion engines of the type which are started by an accessory starting or cranking motor. Such engines are generally provided with spark timing apparatus to generate sparks in the cylinders of the engine at times somewhat in advance of the times when peak cylinder pressure is desired during the power strokes of the pistons in said cylinders. Since combustion within the cylinder takes place at a rate which changes little with varying engine speed, it has been considered desirable to provide a variable spark timing advance with respect to top dead center, which advance increases with engine speed and has been generally known as centrifugal advance. The centrifugal advance has traditionally been added to a certain base engine spark timing which is specified for a particular engine and mechanically fixed by positioning of the distributor during the engine tuning process. This base engine spark timing represents the least possible advance for the typical mechanical engine spark timing system and is calibrated for typical low engine speed such as idle. However, even during idle, the typical internal combustion engine is rotating at 400 revolutions per minute or greater; and the basic timing is therefore often substantially in advance of top dead center.
When such an engine is initially started, it is rotated by an external accessory cranking motor, beginning from a resting condition. Spark events are generated in the cylinders in the normal order from the very beginning of cranking at times determined by the basic timing of the engine. However, the first several such spark events occur with the engine cranking at a speed which is far lower than even the normal idle speed. At such a low speed, a spark event generated at a crank angle of 6.degree. before top dead center, for example, may, under the right conditions in certain engines, produce a cylinder pressure which peaks or at least becomes substantially great before the top dead center position is reached. If this occurs, the pressure of the ignited gases produces a reverse torque which hinders the starting motor in its attempt to accelerate the engine. In fact, since it is most likely to occur on the first engine spark event, it has been known in some cases to prevent the starting motor from starting the engine. This condition has been known as hot stall, since the engine condition most conducive to the problem is that of warm engine temperature. When the engine is warm, the combustion flame front travels faster and the cylinder walls absorb less heat, thus producing an earlier and more powerful peak pressure. In addition, in a warm engine the ignition of the fuel in a cylinder on the very first spark event is much more certain: in a cold engine, on the other hand, the first few ignition events often fail to ignite the fuel mixture and engine speed thus is allowed to increase to a significant value before the first combustion event occurs.
In the past, the easiest solution to such a hot start problem, in the few instances in which it occurred, was to provide a larger, more powerful starting motor. However, as fuel efficiency has assumed greater importance in engine design and every attempt has been made to reduce the weight of motor vehicles in every way possible, it has become desirable to decrease the size of the starting motor and find a different method of solving the hot start problem. One such method has been found in the development of more sophisticated electronically controlled engine spark timing systems in which the actual spark timing is computed relative to an earlier occurring crankshaft synchronized marking pulse which could be set arbitrarily at a point after top dead center. During initial start of the engine, the normal spark timing process could be suppressed in favor of one which generated spark events at the times of the marker pulses themselves.
However, there are still engines using traditional mechanical spark timing control systems which will continue to use such systems for cost or other reasons. It is therefore desirable to provide an improvement in the traditional engine spark timing system which will overcome the problem of hot start in a manner which provides the minimum of costly changes to that system. One approach to this problem is to suppress or prevent ignition at the initiation of engine start or cranking and prevent the ignition events until the engine has built-up sufficient rotational speed to overcome the hot start problem.
To the knowledge of the inventor herein, there have been several publications of apparatus to prevent ignition events during initial starting of a spark ignited internal combustion engine for reasons other than the hot start problem. For example, the U.S. Pat. No. 3,623,464 discloses ignition apparatus for an outboard marine engine in which no spark events are allowed until the engine rotational speed exceeds a predetermined reference. This is done for safety reasons to prevent accidental starting of the engine by someone adjusting or working thereon. This is accomplished by sensing the output of the engine driven alternator as an indication of engine speed. The U.S. Pat. No. 2,398,259 provides a cold start engine starting aid which delays the application of ignition voltage during initial engine cranking for a time determined by a thermostatic timer. In this case, the warmer the engine is, the shorter the delay will become; and, in fact, if the ambient temperature is sufficiently greater than 10.degree. F., there will be no delay at all. U.S. Pat. No. 2,478,739 shows a cold start enhancing ignition system which provides a special high current to the coil during engine start when the battery voltage is lowered by the drain of the starting motor but delays the application of the higher current for a short time by means of a mechanical timer comprising a set of gears so that the higher current will not be applied with possible damage to the coil before the battery voltage is actually lowered.
The preceding references show systems which, although they provide for the delay in the application of spark ignition signals during initial cranking of a spark ignited internal combustion engine, either fail to do so at the most desirable time for the hot start problem or involve cumbersome or otherwise inappropriate apparatus. It is therefore an object of this invention to provide spark timing apparatus for a spark ignited internal combustion engine which aids the starting of a warm engine by preventing the initial ignition events during engine start.
In furtherance of this object, this invention uses a counter to count the initial ignition signals as they occur during engine start and prevents ignition events in response to said signals for a predetermined number of said ignition signals. The use of a counter rather than, for example, a timer is advantageous in that the number of suppressed ignition events is controlled directly and precisely regardless of the performance of the cranking motor; the faster the cranking motor accelerates the engine, the sooner normal ignition is begun. In addition, a counter is not as dependent on temperature and other environmental factors as is an RC circuit or other practical timer in the control of the duration of the suppressed ignition.
Further details and advantages of this invention will be apparent from the accompanying drawings and following description of a preferred embodiment.